Metal golf club head

ABSTRACT

In the metal iron golf club head ( 1 ), the rigidity of the medium sensitivity region ( 5 ) which is a region on the heel side and excluding the vicinity of the face center ( 3 ) is made lower than the rigidity of the low sensitivity region ( 4 ) so as to approximate the natural frequency of the metal iron golf club head to the natural frequency of the ball as well as to ensure the strength in the face portion ( 2 ).

TECHNICAL FIELD

1. The present invention relates to a metal golf club head, and morespecifically, to a configuration of a face portion that improves theinitial speed of the hit ball.

BACKGROUND ART

2. The present inventors have found from the invention described inJapanese Patent Laying-Open No. 7-98077 that the initial speed of thehit ball increases when a golf club head (hereinafter referred to as ahead) is designed to have a natural frequency close to that of the ball.Here, the natural frequency of the head is meant to refer to the naturalfrequency of the face portion at the time it undergoes the impact of ahit. There are various methods of making the head natural frequencysmaller than is conventional to design it to be close to the ballnatural frequency. The most effective method is to lower the rigidity ofthe face portion. Generally, as a method of lowering the rigidity of theface portion, producing a thinner face thickness or using a lowelasticity modulus material may be considered.

3. On the other hand, as a model of an iron golf club, the model havinga face member made of titanium alloy fit into the head body is recentlybecoming popular. The main aim of this model is to achieve an enlargedsweet spot area by utilizing titanium having a light specific gravityfor the face portion so that the head periphery portion becomes heavierin weight. Conversely, some models employ a material having a largespecific gravity such as tungsten for the sole portion in order to lowerthe center of gravity of the head.

4. Moreover, still another model of a metal iron golf club formed by ahead body and a face member made of different materials has an openingthat penetrates from the face to the opposite side of the face and hasthe face member fit into the opening. Thus, the face member is exposedinside the cavity. Titanium alloy is mainly employed as the material forthe face member. The effect achieved by this model is similar to thatachieved by the model in which a plate material made of titanium alloyis fit in as the face member as described earlier. The main objectivehere, again, is the further enlargement of the sweet spot area achievedby exposing the plate material of titanium alloy inside the cavity.Thus, the plate material fit into the opening is generally smooth exceptfor the scoring lines. The plate material itself in the portion exposedinside the cavity does not show much unevenness besides the logoimpression and the like designating the name of the club and thematerial used.

5. Since the properties of titanium alloy involves a light specificgravity as well as a low elasticity modulus, titanium alloy showspromise in that it is a material that can lower the rigidity of the faceportion, which is a requirement for lowering the head natural frequency.The head utilizing titanium alloy for the face member, however,generally has a cavity structure so that the face thickness of thesurface for hitting the ball, though thinner than that of a plain-backtype iron golf club, is considerably thick in comparison with the facesurface of a wood head. In addition, because the iron golf club and thewood golf club have different structures, their natural frequencies inthe range of approximately 1800 Hz to approximately 2000 Hz are largelydifferent from the natural frequencies of the balls in the range ofabout 800 Hz to about 1000 Hz that vary depending on the type of theball.

6. On the other hand, by making the face thickness evenly thin, thestrength of the face portion is reduced, leading to the problem ofdestruction by a hit ball. Thus, as described in Japanese PatentLaying-Open No. 9-192273, there is an invention of a structure in whichthe face thickness around the sweet spot area has a sufficient strengthagainst the impact due to collision between the ball and the faceportion while the face thickness in the region other than the sweet spotarea is made thin.

7. In the head described in Japanese Patent Laying-Open No. 9-192273,however, if the strength of the sweet spot portion on the face isensured while the portion surrounding the sweet spot portion is madethin, the spring performance of the face portion may certainly improve,although at the same time, the impact of the ball hitting the sweet spotwould be supported by the surrounding portion. Since the structure isnot designed to support such an impact, the sweet spot portion of theface portion may possibly disadvantageously cave in in the case of anaverage male golf player hitting the ball with the head speed of about40 m/s.

8. Therefore, the main object of the present invention is to clarify therelation between the rigidity and the natural frequency of the faceportion and to provide a metal golf club head having the smallestpossible head natural frequency while ensuring the strength of the faceportion.

DISCLOSURE OF THE INVENTION

9. The present invention is a golf club head formed of a metal materialand having a medium sensitivity region and a low sensitivity regionprovided on the face portion for hitting a ball, where the facethickness of a part or all of the medium sensitivity region is madethinner than the face thickness in the vicinity of the face center thatexist within the low sensitivity region.

10. According to another aspect, the present invention is a metal golfclub head formed by a head body and a face member formed of differentmaterials and having an opening that penetrates from the face to theopposite side of the face with the face member fit into the opening,where a medium sensitivity region and a low sensitivity region areprovided on the face portion for hitting the ball, with the facethickness of a part or all of the medium sensitivity region made thinnerthan the face thickness of the vicinity of the face center that existswithin the low sensitivity region.

11. According to a still another aspect of the present invention, theface thickness of the face portion is varied so that the face thicknessof a portion which is within the medium sensitivity region and on thesole side from the low sensitivity region and which is a part or all ofthe medium sensitivity region excluding the region extending on eitherside of the face centerline that is within the range of a predefinedlength of the entire length of the face portion in the toe-heeldirection is thinner than the face thickness in the vicinity of the facecenter existing within the low sensitivity region.

12. According to a further aspect of the present invention, a regionwithin the medium sensitivity region and having its face thickness madethin is defined as the low rigidity region, and the area of the lowrigidity region is selected to be 5% to 50% of the face area.

13. According to a still further aspect of the present invention, theregion other than the low rigidity region in the face member is definedas the high rigidity region, and the face thickness of the low rigidityregion is formed at least 0.2 mm thinner than the thickest facethickness of the high rigidity region.

14. According to a still further aspect of the present invention, theface thickness of the low rigidity region is formed at least 0.5 mmthinner than the thickest face thickness of the high rigidity region.

15. Further, the face thickness of the low rigidity region is selectedto be 0.5 to 2.7 mm. In addition, according to a still further aspect ofthe present invention, a material having a tensile strength of 100kgf/mm² or greater and having a Young's modulus of elasticity of 10000kfg/mm² or lower is used for the face member.

16. According to a still further aspect of the present invention, arecessed portion is formed on the face portion from the face of the headtoward the opposite side of the face, and the face member is fit intothe recessed portion.

17. According to a still further aspect, the present invention is ametal golf club head having a hollow portion and formed such that therigidity of a portion on the heel side of the face portion is made lowerthan that on the toe side. The head is formed such that the rigidity ofthe portion excluding the vicinity of the face center is lowered, wherethe region with the lowered rigidity is defined as the low rigidityregion, and the area of the low rigidity region is selected to be in therange of 5% to 50% of the face area.

18. Moreover, the low rigidity region is formed above the face center orin the lower half region on the heel side.

19. According to a still further aspect of the present invention, whenthe region other than the low rigidity region of the face portion isdefined as the high rigidity region, the face thickness of the lowrigidity region is selected to be thinner than the face thickness of thehigh rigidity region. Moreover, the face thickness of the low rigidityregion is formed at least 0.2 mm thinner than the face thickness of thehigh rigidity region, and more preferably, a material having a lowerelasticity modulus than that of the material used in the high rigidityregion is used for the face portion in the low rigidity region.

BRIEF DESCRIPTION OF THE DRAWINGS

20.FIG. 1 is a diagram showing a face portion of a metal iron golf clubhead according to an embodiment of the present invention.

21.FIG. 2 is a diagram showing the results of sensitivity obtained fromthe sensitivity analysis in relation to the face portion of a metal irongolf club head.

22.FIG. 3 is a diagram showing a more specific embodiment in relation tothe face portion of a metal iron golf club head

23.FIG. 4 is a graph representing the measurement results in relation tothe repulsion in the embodiment of FIG. 3.

24.FIG. 5 is a diagram showing a metal golf club head having a hollowportion according to another embodiment of the present invention.

25.FIG. 6 is a diagram showing the results of sensitivity obtained fromthe sensitivity analysis in relation to the face portion of a metal golfclub head having a hollow portion.

26.FIG. 7 is a graph representing the measurement results in relation tothe repulsion in another embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

27. At first, the present inventors performed a sensitivity analysis(sensitivity method) to examine which part of the face portion shouldhave its rigidity lowered in order effectively to lower the naturalfrequency of an iron golf head. The description with regard to thesensitivity analysis will be given below.

28. At first, on the face surface, a hypothetical lattice-like model isprovided with the face center as the starting point and at a spacing of5 mm in the vertical direction (top-sole direction) and at a spacing of10 mm in the horizontal direction (toe-heel direction). With an impulsehammer, vibration is caused in an actual iron club, and the transferfunction expressing the response is obtained. Curve fitting is performedwith the transfer function on a computer to obtain a modal parameter sothat the nature of the effect of a certain vibration on the structurecan be determined.

29. Nobuyuki Ohkubo describes in Machine Modal Analysis (printed May 1,1982) published by Chuo University publishing division, that

30. ‘the sensitivity method involves inputting only the modal parameterof the target machine to output “the sensitive parts” of the structurein relation to the vibration mode or the natural frequency to bealtered. Thus, the alteration in a high sensitivity portion would allowa greater change in the dynamic characteristic with a smaller amount ofalteration.’

31. As described above, the sensitivity analysis is a method ofanalyzing which part of a structure may be effectively changed inrigidity so as to effect a change in the natural frequency of thestructure. Therefore, the value obtained by this analysis is in terms ofsensitivity, and a greater sensitivity value indicates a greater effectof the natural frequency on the change in the rigidity of the structure(i.e. the face surface of the iron according to an embodiment of thepresent invention).

32.FIG. 2 shows the results of the sensitivity analysis of a metal irongolf club head color-coded according to the magnitude of the values.Here, sensitivity is the amount of change in the natural frequencycalculated when adding a unit elasticity modulus (1 N/m) between eachnodal point. For the sake of convenience, the values lower than 15 Hz(N/m) are defined to be of low sensitivity, and the values not smallerthan 15 Hz (N/m) is defined to be of medium sensitivity. The partshaving indicated values not smaller than 25 Hz (N/m) in the region ofthe medium sensitivity is defined as a high sensitivity region.

33. As can be seen from FIG. 2, the region of large sensitivity valuesis distributed in the portions other than the region that extends from acenter top edge 25 of a face portion 2 down to a face center 8.Therefore, it was discovered that, in the case of a metal iron golf clubhead, lowering the rigidity in the portion other than a region 4 thatextends from central top edge 25 down to face center 8 effectivelylowers the natural frequency of a metal iron golf club head 1 so that itbecomes possible to bring it closer to the ball natural frequency. Thus,the region that extends from center top edge 25 of the above-describedface portion 2 down to face center 8 is defined as a low sensitivityregion, and the region other than low sensitivity region 4 is defined asa medium sensitivity region 5.

34. Center top edge 25 is defined as the top edge on a face centerline15 on the face surface.

35. In addition, it was discovered that a region having particularlylarge sensitivity values exists in medium sensitivity region 5.Consequently, it was found that, by lowering the rigidity in such regionhaving particularly large sensitivity values, the natural frequency ofmetal iron golf club head 1 can be lowered effectively so that itbecomes even closer to the natural frequency of the ball. The regionhaving such particularly large sensitivity values is defined as a highsensitivity region 6. As shown in FIG. 2, the region on the toe sidewithin the range of medium sensitivity region 4 is defined as a highsensitivity region 6 a, and the region on the heel side and also withinthe range of medium sensitivity region 4 is defined as a low sensitivityregion 6 b.

36. In other words, the aim of the present inventors, that is, theregion which can influence the head natural frequency, was discovered tobe different from the region around the sweet spot area as described inJapanese Patent Laying-Open No. 9-192273, so that caving-in of the sweetarea by the impact of the ball can be avoided.

37. Thus, with regard to metal iron golf club head 1, it is possible tolower its natural frequency if the rigidity of medium sensitivity region5 shown in FIG. 2 can be lowered.

38. In short, an embodiment of the present invention allows the headnatural frequency to be lowered by optimizing the portion in which therigidity is lowered while preventing the lowering of the strength in theface portion as much as possible by keeping smaller the area of theportion in which the rigidity is to be lowered.

39. To solve the above-described problem, based on the results given inFIG. 2 discovered from the sensitivity analysis of the metal golf clubhead, an embodiment of the present invention has the rigidity lowered inmedium sensitivity region 5 and high sensitivity region 6 which areportions having large sensitivity values so as to lower the head naturalfrequency effectively and thus to improve the repulsion of the headwhile preventing the reduction in the strength of the face poi ion asmuch as possible.

40. An embodiment of the present invention is shown in FIG. 1. As shownin FIG. 1, according to an embodiment of the present invention, in ametal iron golf head 1 formed by a head body 10 and a face member 9formed of different materials and having an opening that penetrates fromthe face to the opposite side of the face with face member 9 fit intothe opening, the invention can be implemented in the form of making therigidity of a part or all of a medium sensitivity region 5 lower thanthe rigidity in the above-described low sensitivity region 4, andspecifically, the invention can be implemented by reducing the facethickness of a part or all of medium sensitivity region 5 to be smallerthan the face thickness in the vicinity of the face center that existsin low sensitivity region 4.

41. In other words, the natural frequency of the head is effectivelylowered without making thinner the face thickness in the vicinity offace center 8 that requires the greatest strength. Moreover, in thisembodiment, as shown in FIG. 1, low sensitivity region 4 refers to theregion enclosed by vertical lines shifted from a face centerline 15toward the toe side and toward the heel side respectively byapproximately 20% of the length of the scoring line from the end pointon the heel side up to the toe, and by a horizontal line shifted from acenter top edge 25 a toward face center 8 by approximately 60% of thelength from the top of face centerline 15 down to the sole, while mediumsensitivity region 5 refers to the region other than the low sensitivityregion 4 of face member 9.

42. Center top edge 25 a is defined as the top edge on face centerline15 of face member 9.

43. In order to produce the effect of this embodiment, there is noparticular limit to the area of the region (hereinafter referred to as alow rigidity region) which is within the range of medium sensitivityregion 5 and in which the face thickness is made thinner as describedabove. In order to produce a distinct effect of this embodiment,however, it is desirable that the low rigidity region has an area notsmaller than 5% of the face area.

44. On the other hand, if the low rigidity region is too large, theportion lacking the strength becomes too large, thereby reducing thestrength of the face as a whole. Therefore, the area of the low rigidityregion is desirably not larger than 70% of the face area, andparticularly, for iron golf club heads for men requiring a certaindegree of strength in the face portion, it is desired that the area ofthe low rigidity region is not larger than 50% of the face area.

45. Moreover, examination of the results of the experiment revealed thathigh sensitivity region 6 which is a region having particularly largesensitivity values did not exist in the vicinity of face centerline 15but was distributed on the toe side and on the heel side. Thus, theinvention can be implemented without making the face thickness of aregion 7, which is defined as the region extending on either side offace centerline 15 in the range of the length which is 5% of the entirelength of face portion 2 in the toe-heel direction, as thin as the facethickness of the remaining medium sensitivity region 5.

46. In addition, this implementation is advantageous in that the headnatural frequency can be somewhat reduced while the head strength can beimproved, although the head natural frequency cannot be reduced as muchas the implementation in which the face thickness of the entire regionof medium sensitivity region 5 is made thin.

47. Further, from the results shown in FIG. 2, the implementation inwhich the rigidity in high sensitivity regions 6 a and 6 b was loweredwas found to be effective. In this embodiment, high sensitivity region 6a is defined as a region within medium sensitivity region 5 as shown inFIG. 1 and located on the toe side from face centerline 15 but excludingregions 7. High sensitivity region 6 b refers to the portion withinmedium sensitivity region 5 as shown in FIG. 1 and located on the heelside and on the sole side from x-axis but excluding region 7.

48. Thus, in order to lower the natural frequency of the head whilelimiting the lowering of the strength of face portion 2 as much aspossible, the implementation in which the rigidity of high sensitivityregion 6 alone is lowered is desirable. Examples of such implementationinclude the implementation in which the rigidity of at least one of highsensitivity region 6 a and high sensitivity region 6 b is lowered, andthe implementation in which the rigidity in both high sensitivityregions 6 a and 6 b is lowered.

49. In addition, the invention can be implemented by making the rigidityof high sensitivity region 6 much lower than the rigidity of mediumsensitivity region 5. More specifically, it is possible to create theface thickness of high sensitivity region 6 thinner than the facethickness of other medium sensitivity region 5, or to employ a lowrigidity material in high sensitivity region 6.

50. There is no particular limit to the implementation in which therigidities of the medium sensitivity region and the high sensitivityregion within the face portion of a metal iron golf club head arelowered, and the present invention can be implemented by making the facethickness in the medium sensitivity region or the like thinner than theface thickness of the low sensitivity region, or by utilizing for themedium sensitivity region and the like a material of a lower elasticitymodulus than the material used in the low sensitivity region.

51. Here, normally the natural frequency of an iron head formed by asingle material is around 2000 Hz. In the iron head having a platematerial of titanium alloy inserted in the face portion, a type of aniron head becoming popular in recent years, the natural frequency isabout 1800 Hz. In either case, the head natural frequency is quitedifferent from the natural frequency of a ball in the range of 800 to1000 Hz. In order to abridge the gap of approximately 100 Hz betweenthese natural frequencies, a desirable method should allow the naturalfrequency to be lowered in relation to both the face thickness and thematerial.

52. Therefore, in this embodiment, the head body and the face member aremade of different materials, and a material having a low elasticitymodulus is used as the material for the face member to lower therigidity of the entire face portion, thereby taking advantage of theeffect of the embodiment more fully. When different materials are usedfor the head body and the face member to form the iron golf club head,stainless steel is generally used for the head body while titanium alloyis most suitably used for the face member.

53. Further, when the invention is implemented for the head having anopening that penetrates from the face to the opposite side of the face,the rigidity of the entire face portion can be further lowered so thatthe head natural frequency can be brought even closer to the ballnatural frequency. It is also effective to increase the area of exposureinside the cavity of the face member in order to lower the rigidity ofthe face portion.

54. Moreover, according to the results from the sensitivity analysis,that is, due to the fact that high sensitivity legions 6 a and 6 b havelarge sensitivity values, the regions on the backside of face member 9and corresponding to high sensitivity regions 6 a and 6 b are preferablyexposed inside the cavity, allowing the rigidity to be further lowered.

55. In this case, there is no limitation as to the material to be usedfor the face member, and the invention may be implemented using variousmaterials, such as stainless steel, titanium alloy, aluminum alloy, andmagnesium alloy. From the aspects of strength and lowness of elasticitymodulus, either α-type or β-type titanium alloy is suitable for facemember 9, and especially β-type titanium alloy having high strength anda low elasticity modulus is desirable. More specifically, β-typetitanium alloy having a tensile strength of kgf/mm² or greater andhaving a Young's modulus of elasticity of 10000 kfg/mm² or lower ispreferred.

56. In short, in order fully to bring out the effect of this embodiment,it is desirable to implement the invention for a metal iron golf clubhead formed by a head body and a face member, and having an opening thatpenetrates from the face to the opposite side of the face, with facemember 9 fit into the opening.

57. Here, in order to lower the rigidity of the portion which relates tothe lowering of the natural frequency of the head in the most effectivemanner as discovered by the sensitivity analysis as described above, themethod of making the face thickness of that portion thinner issuggested.

58. In the implementation in which the face thickness of the mediumsensitivity region is made thinner than the face thickness of the lowsensitivity region, the face thickness of the low rigidity region withinthe medium sensitivity region having its face thickness made thinner andthe face thickness of a high rigidity region within the low sensitivityregion should be determined by the material used for the face portionand by the head speed of the target golf player, and thus, are notlimited to any particular values. In other words, since the object ofthis embodiment lies in bringing the natural frequency of the head closeto that of the ball, the degree of face thickness in the low rigidityregion and the high rigidity region is to be determined in relation tosuch factors as the face material, the face shape, and the area of theface.

59. Next, as a more specific embodiment of the present invention, thereis no particular limit to the face thickness of the high rigidityregion, and the value of the face thickness is determined by thematerial used. More specifically, as with a normal golf club, withtitanium alloy or stainless steel the face thickness can be implementedin the range of 2.0 to 5.0 mm while with aluminum alloy or magnesiumalloy the face thickness may be implemented in the range of 2.5 to 8.0mm. When considering only the effect of lowering the head naturalfrequency, it can be said that a thin face thickness is desirable alsoin the high rigidity region. The high rigidity region, however, is apart requiring the strength to endure the impact of the hit ball.Therefore, when using titanium alloy having a high strength for thematerial to form the face member, face thickness of about 2.5 mm toabout 3.0 mm would be suitable for the high rigidity region. Moreover,when using aluminum alloy for the face member, face thickness of about3.0 mm to about 4.0 mm would be appropriate.

60. If the face thickness of the low rigidity region is too thick, theeffect of lowering the head natural frequency cannot be achieved. Thus,in the case of a male player whose normal head speed is about 40 m/s,the face thickness of the low rigidity region is preferably not greaterthan 2.7 mm. On the other hand, a face thickness that is too thin cancreate disadvantages such as increased difficulty during the polishingprocess and poor production yield. Consequently, the face thickness ofthe low rigidity region should be not smaller than 0.7 mm.

61. When titanium alloy is used as the material for the face member, theface thickness of the low rigidity region must be within the range of0.7 to 2.7 mm for the same reason.

62. Moreover, in this embodiment, the natural frequency of the head canbe brought closer to that of the ball by making the face thickness ofthe low rigidity region thinner than the face thickness of the highrigidity region so as to limit the reduction of the head strength asmuch as possible while lowering the natural frequency of the head.Accordingly, it can be said that a thinner face thickness in the lowrigidity region is more desirable.

63. If, however, the difference in face thickness between these regionsis too small, a distinct effect of this embodiment cannot be achieved.Thus, the difference in face thickness between the low rigidity regionand the high rigidity region should be not smaller than 0.2 mm, andfurther, a desirable difference in face thickness is not smaller than0.5 mm to ensure a distinct effect of this embodiment.

64. Thus, according to one embodiment of the present invention, the facethickness of the high rigidity region can be implemented to be in therange of 2.0 to 5.0 mm when using titanium alloy or stainless steel forthe face member, or in the range of 2.5 to 8.0 mm when using aluminumalloy or magnesium alloy, while the face thickness of the low rigidityregion is implemented in the range of 0.7 to 2.7 mm, with the differencebetween the face thickness of the high rigidity region and the facethickness of the low rigidity region not smaller than 0.2 mm. Further,the desirable difference in face thickness is not smaller than 0.5 mm.

65. Furthermore, in the case of golf clubs intended for golf playerswith slow head speeds such as those intended for women, while it ispossible to reduce the strength of the high rigidity region and the lowrigidity region further than usual, at the same time, clubs having alighter weight than normal golf clubs must be produced. Thus, the facethickness of the high rigidity region can be implemented in the range of1.5 to 4.0 mm when using titanium alloy or stainless steel for the facemember, or in the range of 1.5 to 5.0 mm when using aluminum alloy ormagnesium alloy, and the face thickness of the low rigidity region canbe implemented in the range of 0.5 to 2.5 mm, with the difference inface thickness between low rigidity region and the high rigidity regionnot smaller than 0.2 mm.

66. Therefore, when the golf clubs for women are also taken intoconsideration, the face thickness of low rigidity region can beimplemented in the range of 0.5 to 2.7 mm.

67. In addition, this embodiment can be implemented for all types ofmetal iron golf club heads, regardless of the head shape, the loftangle, the head weight, or the club type. Thus, it can be implementedwith a No. 1 iron down to a No. 9 iron as well as with a PW, an SW, anAW, an FW and the like.

68. Now, a metal iron golf dub head according to an embodiment of thepresent invention will be described using FIG. 3. FIG. 3 is a diagramshowing the face portion and the face member of a metal iron golf dubhead, with the enlarged view of the face member representingparticularly the face member seen from the front, while the differenceof face thickness is expressed by the tone. That is, in the diagram, thecolored portion represents low rigidity region (11) where the facethickness is thin, and the non-colored portion represents high rigidityregion (12).

69. In addition, a face center 8 denotes the intermediate point betweenthe top and the bottom of face member 9 along the extended line of aface centerline 15.

70. Moreover, needless to say, since face center 8 is represented by avalue which greatly influences the shape and the material of the clubhead, different values must be considered corresponding to the differenttypes of dub heads.

71. The embodiment shown in FIG. 3 is an embodiment of a No. 5 iron.Stainless steel was used as the material for head body 10, and β-typetitanium alloy, among titanium alloys, which has a high strength and alow elasticity modulus, was used for face member 9. The face shape wasas shown in FIG. 3, with the face area being 38.7 cm², the head lengthbeing 88 mm, and the head weight being 261 g. In addition, face member 9was constructed such that it was exposed inside the cavity, with thearea of the cavity portion being 32 cm².

72. The average value of the face thickness of high rigidity region 12was set to the same value as that of a conventional product, i.e. 3.0mm. Three different values were used for the face thickness of lowrigidity region 11, which were 2.5 mm, 2.0 mm, and 1.0 mm. Embodiment 1utilized the face thickness of 2.5 mm for low rigidity region 11,embodiment 2 utilized the face thickness of 2.0 mm, and embodiment 3utilized the face thickness of 1.0 mm. Here, low rigidity regions 11 aand 11 b were defined as follows. First, as shown in FIG. 3, thecoordinates of face center 8 was set to the point (0, 0) (unit: mm).With the x-axis being the toe-to-heel direction (heel direction beingpositive) and the y-axis being the top-to-sole direction (top directionbeing positive), the region enclosed by the curved line connecting thecoordinates A (−38, 16) and the coordinates B (−15, −15) and having R ofabout 40 mm and by the outer circumference of the cavity was defined aslow rigidity region 11 a, and the region enclosed by the curved lineconnecting the coordinates C (15, −17) and the coordinates D (25, 5) andhaving R of about 30 mm and by the outer circumference of the cavity wasdefined as low rigidity region 11 b.

73. Moreover, as comparative samples, two kinds of conventional productswere prepared: a commercial product having a head formed of a singlematerial of soft iron and a commercial product utilizing titanium alloyin the face portion but not having it exposed inside the cavity (withthe thickness of the member forming the face portion being 3.0 mm). Theiron head using soft iron is referred to as a conventional product 1,and the commercial product utilizing titanium alloy in the face portionbut not having it exposed inside the cavity is referred to as aconventional product 2.

74. Table 1 shows the measured results of the natural frequencies of theembodiments and the conventional products. TABLE 1 Thickness of lowDifference in Thickness of rigidity thickness of high Natural highrigidity region and low rigidity Frequency region (mm) (mm) regions (mm)(Hz) Conventional 3.1 — — 2037.5 product 1 Conventional 4.5  — — 1850.0product 2 Embodiment 1 3.0 2.5 0.5 1210.0 Embodiment 2 3.0 2.0 1.01145.0 Embodiment 3 3.0 1.0 2.0  895.0

75. It can be seen from the table that the value of the naturalfrequency of embodiment 3 is the smallest among the embodiments, andthat the natural frequency becomes lower as the face thickness of lowrigidity region 11 becomes thinner.

76. It is also seen that the respective natural frequencies ofembodiments 1, 2 and 3 having their face thickness in low rigidityregion 11 made thinner are drastically lowered in comparison with therespective natural frequencies of conventional products 1 and 2 (2038 Hzfor conventional product 1, 1850 Hz for conventional product 2).Considering that the natural frequency of the golf ball, although it maymore or less vary from kind to kind, is within the range of about 800 Hzto about 1000 Hz, the inventors were successful in realizing in theseembodiments the values of the natural frequencies which are in about thesame range as the natural frequency of a golf ball.

77. From the above, in the attempt to bring the natural frequency of thehead closer in range to the natural frequency of the ball by making theface thickness of low rigidity region 11 thin, it can be concluded thatsufficient effect was observed by forming the face thickness of lowrigidity region 11 at least 0.5 mm thinner than high rigidity region 12.The fact that forming the face thickness of low rigidity region 11 atleast 0.5 mm thinner than high rigidity region 12 has proved effective,however, is only applicable to the head implementation according to thepresent embodiment. In the case of a head utilizing a material having alower elasticity modulus than that used in the present embodiment or inthe case of a head having an enlarged face area, the head naturalfrequency can be brought closer to the ball natural frequency with thedifference in face thickness between the high rigidity region and thelow rigidity region being as small as about 0.2 mm.

78. Then, in addition to the measurement of the natural frequenciesdescribed earlier, tests relating to repulsion were carried out. Theheads used for the testing were the above-described embodiments 1, 2,and 3, and conventional products 1 and 2.

79. The tests carried out here in relation to repulsion involvedshooting the ball against the tested heads at a speed of about 37 m/s,which is about the same speed as the 5I head speed of an average golfplayer, in order to evaluate the quality of repulsion from this speedand the speed at which the ball bounces back. The heads implementing thepresent invention and the conventional product heads were tested.

80. The results are shown in FIG. 4 as the head repulsion ratios.

81. The repulsion ratio was calculated as follows:

82. Repulsion ratio=shot-out speed of ball/head speed=(ball reflectionrate+ball impingement rate)/ball impingement rate

83. As can be seen from FIG. 4, the repulsion ratios for all theembodiments are higher in comparison with those for the conventionalproducts. Moreover, among the embodiments, the repulsion ratio ofembodiment 3 indicated the largest value. It was found that anembodiment having a thin face thickness in the low rigidity region andhaving a natural frequency close to that of the ball natural frequencyhad a good repulsion ratio.

84. As described above, according to an embodiment of the presentinvention, in a metal iron golf club head being formed by a head bodyand a face member made of different materials and having an opening thatpenetrates from the face to the opposite side of the face, by formingthe face thickness of a part or all of medium sensitivity region 5thinner than the face thickness of an average value in low sensitivityregion 4, the natural frequency of the head can be brought closer to thenatural frequency of the ball without reducing the strength of the face,while improving the repulsion between the head and the ball.

85. Next, the sensitivity analysis was performed to examine which partof the face portion should have its rigidity lowered in order to lowerthe natural frequency of another model of a golf club, i.e. the headhaving a hollow portion. The results for a metal golf club head having ahollow portion are shown in FIG. 6. In handling the results, themagnitude of the value of sensitivity obtained by the sensitivityanalysis becomes the indices indicating the magnitude of the influenceto be effected on the natural frequency. In other words, the naturalfrequency of the head can be more effectively lowered by lowering therigidity of a portion having large sensitivity values. FIG. 6color-codes the magnitudes of the sensitivity values according to theresults of such sensitivity analysis.

86. As seen from FIG. 6, the region having large sensitivity values liesin the region on the heel side 37 of a face centerline 34 and excludingthe vicinity of a face center 33. Thus, in the case of a metal golf clubhead having a hollow portion, it was found that the natural frequency ofthe golf head can be effectively lowered and be brought closer to thenatural frequency of the ball by lowering the rigidity in the portion onheel side 37 of face centerline 34 and excluding the vicinity of facecenter 33.

87. Therefore, in order to lower the head natural frequency, it wasdiscovered not always to be appropriate to require the thickness of theentire portion other than the vicinity of the sweet area to be made thinas described in the above-mentioned Japanese Patent Laying-Open Nos.9-168613 and 9-192273.

88. Thus, the natural frequency of a metal golf club head having ahollow portion can be lowered if the rigidity of the region on the heelside of the face centerline of the face portion and excluding thevicinity of the sweet area, as shown in FIG. 6, can be lowered.

89. It can be seen from FIG. 6 that the portion having large sensitivityvalues is located mainly on heel side 37. Therefore, it was found thatmaking the rigidity in a portion of heel side 37 lower than that in atoe side 36 would prove effective. Here, the heel side refers to heelside 37 on the right side of face centerline 34 as shown in FIG. 5. Itwas also discovered that, since the sensitivity values of toe side 36are extremely small, lowering the rigidity in this portion would not beso effective in lowering the natural frequency of the head. Here, thetoe side refers to toe side 36 on the left side of face centerline 34 asshown in FIG. 5.

90. Thus, this embodiment can be implemented by making the rigidity in aportion of the heel side lower than the rigidity of the toe side.

91. Further, in producing the effect of this embodiment, there is noparticular limit as to the area of the region in which the rigidity ismade lower (hereinafter referred to as a low rigidity region). In orderto produce a distinct effect of this embodiment, however, it isdesirable that the region in which the rigidity is lowered has an areathat is not smaller than 5% of the face area.

92. On the other hand, if the low rigidity region is too large, theportion lacking the strength becomes too large, thereby reducing thestrength of the face as a whole. Therefore, the area of the low rigidityregion is desirably not larger than 50% of the face area.

93. Moreover, examination of the results of the experiment revealed thatthe sensitivity values around face center 33 were small. In addition,this region is the so-called sweet area which undergoes the impact ofthe ball, and thus requires sufficient strength to endure the impact.From these two points, it was concluded that there was no need to lowerthe rigidity in the region around face center 33.

94. Furthermore, since the sensitivity values in a region 51 shown inFIG. 6 are large, the implementation in which the rigidity of suchregion 51 is lowered was also found to be effective. Here, region 51refers to the region on heel side 37 and on the sole side of face center33, and located substantially midway between face centerline 34 and heel39, as shown in FIG. 6.

95. In addition, since the sensitivity values in a region 52 shown inFIG. 6 are also large, the implementation in which the rigidity of suchregion 52 is lowered was also found to be effective. Here, region 52refers to a region within a larger region on heel side 37 and on thecrown side of face center 33, and when the larger region thus defined isdivided generally in half in the crown-sole direction, region 52 islocated in the region closer to the crown side and in the vicinity ofthe face centerline, as shown in FIG. 6.

96. Since the portions having particularly high sensitivity values areregions 51 and 52, the head natural frequency is more effectivelylowered by implementing both of these regions as low rigidity regions.

97. Thus, there is no particular limitation as to the location of thelow rigidity region as long as it is within the range of the heel sideof the face portion.

98. In the face portion of the metal golf club head having a hollowportion, there is no limitation as to how the rigidity of the lowrigidity region is lowered. Thus, the thickness of the low rigidityregion may be made thinner than the thickness of the region other thanthe low rigidity region (hereinafter referred to as a high rigidityregion), or a material having a lower elasticity modulus than that ofthe material used in the high rigidity region may be utilized for thelow rigidity region.

99. The invention can also be implemented by attaching a material havinga high elasticity modulus on the back of the face portion of the highrigidity region after having formed the head to have an even facethickness.

100. As regards the cost, it is desirable that this embodiment isimplemented by forming the thickness of low rigidity region thinner thanthe thickness of the high rigidity region.

101. On the other hand, it is also effective to make the difference inthe elasticity modulus of the materials used in the high rigidity regionand the low rigidity region greater in order to make the difference inrigidity between the two regions greater. In such a case, the use of amaterial having a lower elasticity modulus than that of the materialused in the high rigidity region for the low rigidity region ispreferred.

102. Further, regarding the ease of construction, the implementation inwhich a high elasticity modulus material is attached on the back of theface portion of the high rigidity region is preferred.

103. In the case in which the thickness of the low rigidity region ismade thinner than the thickness of the high rigidity region, the valuesof the thickness of the face portion and of the difference in thicknessbetween the low rigidity region and the high rigidity region should bedetermined by the material used for the face portion and by the headspeed of the target golf player, and thus, are not limited to anyparticular values.

104. In other words, since the object of this embodiment lies inbringing the natural frequency of the head close to that of the ball,the thickness of the low rigidity region is to be determined in relationto such factors as the face material; the thickness of the face portion;that is, of the portion that serves as the high rigidity region in thisembodiment; the area of the face; and the shape of the head.

105. Here, as a material for forming the face, titanium alloy havinghigh strength and a low elasticity modulus, is desirable, for instance,since titanium alloy allows the face portion to be made thin. Normally,the thickness of the face portion formed with an even thickness is inthe range of 2.5 to 3.0 mm for a 1 W. According to this embodiment,however, this thickness is only applied to the high rigidity region,while the thickness of the low rigidity region is made at least 0.2 mmthinner. As a result, the rigidity in the face portion can be furtherlowered, and the natural frequency of the head can be brought closer tothat of the ball.

106. Since the results obtained from the sensitivity analysis depend onthe structure of the golf club head and are not limited to the materialused as long as it is a metal golf club head having a hollow portion,the present embodiment is applicable to metal golf club heads formedwith stainless steel, aluminum alloy, or the like besides theabove-described titanium alloy. Even in this case, the difference inthickness of at least 0.2 mm is required, as was the case with titaniumalloy, in order to achieve a distinct effect of the present invention.

107. For instance, in the case of golf clubs intended for golf playerswith slow head speeds such as those intended for women, since thestrength of the low rigidity region can be lowered than usual, thedifference in thickness between the low rigidity region and the highrigidity region is preferably at least 0.5 mm.

108. If the face thickness of the low rigidity region is too thick, theeffect of lowering the head natural frequency cannot be achieved. Thus,in the case of a male player whose normal head speed is about 40 m/s,with stainless steel being the material used for the face portion, theface thickness of the low rigidity region is preferably not greater than2.6 mm. On the other hand, a face thickness that is too thin can createdisadvantages such as increased difficulty during the polishing processand poor production yield. Consequently, the face thickness of the lowrigidity region should be not smaller than 0.5 mm.

109. Moreover, when titanium alloy is used as the material for the faceportion, the face thickness of the low rigidity region must be withinthe range of 0.5 to 2.5 mm for the same reason.

110. The difference in the thickness of the low rigidity region and thehigh rigidity region has thus far been discussed above. Whenimplementing the present invention with a golf club head having a faceportion formed with an even thickness, the thickness of the highrigidity region may be about the same as that of a commerciallyavailable model. That is to say, in the case of a 1W, the desirablethickness when using titanium alloy is about 2.5 to about 3.0 mm, asdescribed earlier, and the desirable thickness when using stainlesssteel is about 2.6 to about 2.8 mm.

111. In addition, when utilizing for the low rigidity region a materialhaving a lower elasticity modulus than that of the material used in thehigh rigidity region, the material used for the face portion is notlimited to any particular material as long as it is a material normallyused for a golf head. In other words, the face portion may beimplemented using stainless steel, aluminum alloy, titanium alloy, orthe like.

112. Since the high rigidity region includes a sweet spot and thus isrequired to have high strength, the suitable material for the highrigidity region would be titanium alloy or stainless steel. On the otherhand, the low rigidity region requires a small elasticity modulus aswell as a certain degree of strength- Therefore, the use of α-typetitanium alloy such as pure titanium and 6-4 titanium for the highrigidity region in combination with the use of β-type titanium or thelike for the low rigidity region, or the use of stainless steel for thehigh rigidity region in combination with the use of titanium alloy forthe low rigidity region is suitable.

113. Further, for a high head speed golf player, the use of titaniumalloy for the low rigidity region in combination with the use ofstainless steel for the high rigidity region is desirable, while for alow head speed golf player, the use of aluminum alloy for the lowrigidity region in combination with the use of titanium alloy in thehigh rigidity region is desirable.

114. The face thickness of the low rigidity region need not be the sameas the face thickness of the high rigidity region. Considering the easeof construction, however, it is preferable that the face thicknesses areequal. In order to enlarge the difference of rigidity between theseregions, the low rigidity region is preferably formed by a lowelasticity modulus material, and further, the face thickness of the lowrigidity region is made thin.

115. In addition, this embodiment can be implemented in all types ofmetal iron golf club heads having a hollow portion, without limitationas to the head shape, the loft angle, the head weight, or the club type.Thus, it can be implemented with a 1W, a 2W, a 3W, a 4W, a 5W, and thelike.

116. An embodiment of a metal golf club head according to the presentinvention will be described in relation to FIG. 1.

117.FIG. 5 is a diagram showing an embodiment of the present inventionin relation to the face portion of a metal golf club head having ahollow portion.

118. In this embodiment, the sweet area was designed as an ellipsehaving a major axis of about 20 mm and a minor axis of about 10 mm witha face center 3 serving as its center. In addition, face center 3designates the intermediate point between the top and the bottom of faceportion along the extended line of a face centerline 4.

119. Needless to say, since the sweet area and the face center arerepresented by values which greatly influence the shape and the materialof the club head, different values must be considered corresponding tothe different types of club heads.

120. The embodiment shown in FIG. 5 employs titanium alloy as amaterial. The face shape is as shown in FIG. 1, with the face area being30 cm², the face length being 95 mm, the face height being 44.5 mm, andthe head weight being 207 g.

121. The face thickness of high rigidity region 40 was set to the samevalue as a conventional product, i.e. 2.8 mm. Three different valueswere used for the face thickness of low rigidity region 35, which were2.3 mm, 1.8 mm, and 1.3 mm. Embodiment 4 utilized the face thickness of2.3 mm for low rigidity region 35, embodiment 5 utilized the facethickness of 1.8 mm, and embodiment 6 utilized the face thickness of 1.3mm.

122. Here, as shown in FIG. 5, with the coordinates of face center 33set to the point (0, 0) (unit: mm) where the x-axis is the directionfrom the toe 38 to the heel 39 of a metal golf club head 31 (heeldirection being positive) and the y-axis is the direction from the crown41 to the sole 42 (crown direction being positive), the low rigidityregion refers to the region on the heel side 37 from the line connectingthe coordinates A (5, 20), B (5, 10), C (15, 5), D (20, 0), E (15, −5),F (−5, −10), and G (−5, −20), represented as the shaded portion in FIG.5.

123. Moreover, as a comparative sample, a conventional product made ofthe same titanium alloy, having the face thickness of 2.8 mm, and havingthe same shape and approximately the same weight as each of theinventive metal golf dub head product was prepared.

124. Table 2 shows the measured results of the natural frequencies ofthe embodiments and the conventional metal golf club head product. TABLE2 Thickness of Difference in thickness low rigidity of high and lowrigidity Natural region (mm) regions (mm) frequency Conventional 2.8 —1190.0  product Embodiment 4 2.3 0.5 990.0 Embodiment 5 1.8 1.0 890.0Embodiment 6 1.3 1.5 797.5

125. It can be seen from Table 2 that the natural frequency is smallerin an inventive product having a thinner face thickness in low rigidityregion 35. Therefore, it can be concluded that the natural frequency ofthe metal golf club head is lowered as the face thickness of lowrigidity region 5 becomes thinner, and thus, as the rigidity is lowered.

126. It is also seen that, in the embodiments having the face thicknessof their respective low rigidity regions made thinner, the respectivenatural frequencies are drastically lowered in comparison with thenatural frequency of the conventional products, i.e. 1190 Hz.Considering that the natural frequency of the golf ball, although it maymore or less vary from kind to kind, is within the range of about 800 Hzto about 1000 Hz, the inventors were successful in realizing in theseembodiments the values of the natural frequencies which are in about thesame range as the natural frequency of a golf ball.

127. From the above, in the attempt to bring the natural frequency ofthe head closer in range to the natural frequency of the ball by makingthe face thickness of the low rigidity region thin, it can be concludedthat sufficient effect was observed by forming the face thickness of thelow rigidity region at least 0.5 mm thinner than the high rigidityregion.

128. The fact that forming the face thickness of the low rigidity regionat least 0.5 mm thinner than the high rigidity region has provedeffective, however, is only applicable to the head implementationaccording to the present embodiment. In the case of a head utilizing amaterial having a lower elasticity modulus than that used in the presentembodiment or in the case of a head having an enlarged face area, thehead natural frequency can be brought closer to the ball naturalfrequency with the difference in thickness between the high rigidityregion and the low rigidity region being as small as about 0.2 mm.

129. Then, in addition to the measurement of the natural frequenciesdescribed earlier, tests relating to repulsion were carried out.

130. The tests carried out here in relation to repulsion involvedshooting the ball against the tested heads at a speed of about 40 m/s,which is about the same speed as the driver head speed of an averagegolf player, in order to evaluate the quality of repulsion from thisspeed and the speed at which the ball bounces back. The headsimplementing the present invention and a conventional product head weretested.

131. The results are shown in FIG. 7 as the repulsion ratios of both theinventive and conventional heads.

132. Here, the repulsion ratio was calculated in the same manner as thatof the metal iron golf club heads described earlier.

133. As can be seen from FIG. 7, the repulsion ratios for all theinventive products are higher in comparison with that of theconventional product. Moreover, among the embodiments, the repulsionratio of embodiment 3 indicated the largest value. It was found that anembodiment having a thinly formed low rigidity region and having anatural frequency close to that of the ball natural frequency had a goodrepulsion ratio.

134. As described above, according to the present invention, in a metaliron golf club head having a hollow portion, by reducing the rigidity inthe portion on the heel side of the head portion, the natural frequencyof the head can be brought closer to the natural frequency of the ballwithout reducing the strength of the face, while improving the repulsionbetween the head and the ball.

Industrial Applicability

135. As seen from the above, according to the present invention, byforming the face thickness of a part or all of the medium sensitivityregion of the face member of a metal golf club head thinner than theface thickness of an average value in the low sensitivity region, thenatural frequency of the metal golf club head can be brought closer tothe natural frequency of the ball without reducing the strength of theface, while improving the repulsion between the head and the ball.

CLAIMS:
 1. A metal golf club head formed of a metal material, wherein amedium sensitivity region (5) and a low sensitivity region (4) areprovided on a face portion (2) for hitting a ball, and face thickness ofa part or all of said medium sensitivity region (5) is made thinner thanface thickness in a vicinity of a face center (8) that exists withinsaid low sensitivity region (4).
 2. The metal golf club head accordingto claim 1 , wherein face thickness of face portion (2) is varied sothat face thickness of a portion which is within said medium sensitivityregion (5) and toward a sole side from said low sensitivity region (4)and which is a part or all of said medium sensitivity region (5)excluding a region (7) extending on either side of a face centerline(15) that is within a range of a predefined length of entire length offace portion (2) in the toe-heel direction is thinner than facethickness in a vicinity of a face center existing within said lowsensitivity region (4).
 3. The metal golf club head according to claim 1, wherein a region within said medium sensitivity region (5) and havingits face thickness made thin is defined as a low rigidity region (1),and area of said low rigidity region (11) is 5% to 50% of face area. 4.The metal golf club head according to claim 2 , wherein a region withinsaid medium sensitivity region (5) and having its face thickness madethin is defined as a low rigidity region (11), and area of said lowrigidity region (11) is 5% to 50% of face area.
 5. The metal golf clubhead according to claim 3 , wherein a region other than low rigidityregion (11) in said face portion (2) is defined as a high rigidityregion (12), and face thickness of said low rigidity region (11) isformed at least 0.2 mm thinner than thickest face thickness of said highrigidity region (12).
 6. The metal golf club head according to claim 4 ,wherein a region other than low rigidity region (11) in said faceportion (2) is defined as a high rigidity region (12), and facethickness of said low rigidity region (11) is formed at least 0.2 mmthinner than thickest face thickness of said high rigidity region (12).7. The metal golf club head according to claim 3 , wherein facethickness of said low rigidity region (11) is formed at least 0.5 mmthinner than thickest face thickness of said high rigidity region (12).8. The metal golf club head according to claim 4 , wherein facethickness of said low rigidity region (11) is formed at least 0.5 mmthinner than thickest face thickness of said high rigidity region (12).9. The metal golf club head according to claim 3 , wherein facethickness of said low rigidity region (11) is selected to be 0.5 to 2.7mm.
 10. The metal golf club head according to claim 4 , wherein facethickness of said low rigidity region (11) is selected to be 0.5 to 2.7mm.
 11. The metal golf club head according to claim 1 , wherein amaterial having a tensile strength of 100 kgf/mm² or greater and havinga Young's modulus of elasticity of 10000 kfg/mm² or lower is used forsaid face member (2).
 12. The metal golf club head according to claim 2, wherein a material having a tensile strength of 100 kg/mm² or greaterand having a Young's modulus of elasticity of 10000 kfg/mm² or lower isused for said face member (2).
 13. The metal golf club head according toclaim 3 , wherein a material having a tensile strength of 100 kgf/mm² orgreater and having a Young's modulus of elasticity of 10000 kfg/mm² orlower is used for said face member (2).
 14. The metal golf club headaccording to claim 4 , wherein a material having a tensile strength of100 kgf/mm² or greater and having a Young's modulus of elasticity of10000 kfg/mm² or lower is used for said face member (2).
 15. The metalgolf club head according to claim 5 , wherein a material having atensile strength of 100 kgf/mm² or greater and having a Young's modulusof elasticity of 10000 kfg/mm² or lower is used for said face member(2).
 16. The metal golf club head according to claim 6 , wherein amaterial having a tensile strength of 100 kgf/m/m² or greater and havinga Young's modulus of elasticity of 10000 kfg/mm² or lower is used forsaid face member (2).
 17. The metal golf club head according to claim 7, wherein a material having a tensile strength of 100 kgf/mm² or greaterand having a Young's modulus of elasticity of 10000 kfg/mm² or lower isused for said face member (2).
 18. The metal golf club head according toclaim 8 , wherein a material having a tensile strength of 100 kgf/mm² orgreater and having a Young's modulus of elasticity of 10000 kfg/mm² orlower is used for said face member (2).
 19. The metal golf club headaccording to claim 9 , wherein a material having a tensile strength of100 kgf/mm² or greater and having a Young's modulus of elasticity of10000 kfg/mm² or lower is used for said face member (2).
 20. The metalgolf club head according to claim 10 , wherein a material having atensile strength of 100 kgf/mm² or greater and having a Young's modulusof elasticity of 10000 kfg/mm² or lower is used for said face member(2).
 21. The metal golf club head according to claim 1 , wherein arecessed portion is formed on said face portion (2) from face of a headtoward opposite side of the face, and face member (9) is fit into saidrecessed portion.
 22. The metal golf club head according to claim 2 ,wherein a recessed portion is formed on said face portion (2) from faceof a head toward opposite side of the face, and face member (9) is fitinto said recessed portion.
 23. The metal golf club head according toclaim 3 , wherein a recessed portion is formed on said face portion (2)from face of a head toward opposite side of the face, and face member(9) is fit into said recessed portion.
 24. The metal golf club headaccording to claim 4 , wherein a recessed portion is formed on said faceportion (2) from face of a head toward opposite side of the face, andface member (9) is fit into said recessed portion.
 25. The metal golfclub head according to claim 5 , wherein a recessed portion is formed onsaid face portion (2) from face of a head toward opposite side of theface, and face member (9) is fit into said recessed portion.
 26. Themetal golf club head according to claim 6 , wherein a recessed portionis formed on said face portion (2) from face of a head toward oppositeside of the face, and face member (9) is fit into said recessed portion.27. The metal golf club head according to claim 7 , wherein a recessedportion is formed on said face portion (2) from face of a head towardopposite side of the face, and face member (9) is fit into said recessedportion.
 28. The metal golf club head according to claim 8 , wherein arecessed portion is formed on said face portion (2) from face of a headtoward opposite side of the face, and face member (9) is fit into saidrecessed portion.
 29. The metal golf club head according to claim 9 ,wherein a recessed portion is formed on said face portion (2) from faceof a head toward opposite side of the face, and face member (9) is fitinto said recessed portion.
 30. The metal golf club head according toclaim 10 , wherein a recessed portion is formed on said face portion (2)from face of a head toward opposite side of the face, and face member(9) is fit into said recessed portion.
 31. A metal golf club head formedby a head body and a face member and the like formed of differentmaterials and having an opening that penetrates from face to oppositeside of the face with a face member (9) fit into the opening, wherein amedium sensitivity region (5) and a low sensitivity region (4) areprovided on a face portion (2) for hitting a ball, and face thickness ofa part or all of said medium sensitivity region (5) is made thinner thanface thickness in a vicinity of a face center (8) that exists withinsaid low sensitivity region (4).
 32. The metal golf club head accordingto claim 31 , wherein face thickness of said face member (9) is variedso that face thickness of a portion which is within said mediumsensitivity region (5) and toward a sole side from said low sensitivityregion (4) and which is a part or all of said medium sensitivity region(5) excluding a region (7) extending on either side of face centerline(15) that is within a range of a predefined length of entire length offace portion (2) in the toe-heel direction is thinner than facethickness in a vicinity of a face center existing within said lowsensitivity region (4).
 33. The metal golf club head according to claim31 , wherein a region within said medium sensitivity region (5) andhaving its face thickness made thin is defined as a low rigidity region(11), and area of said low rigidity region (11) is 5% to 50% of facearea.
 34. The metal golf club head according to claim 32 , wherein aregion within said medium sensitivity region (5) and having its facethickness made thin is defined as a low rigidity region (11), and areaof said low rigidity region (11) is 5% to 50% of face area.
 35. Themetal golf club head according to claim 33 , wherein a region other thansaid low rigidity region (1 1) in said face member (9) is defined as ahigh rigidity region (12), and face thickness of said low rigidityregion (11) is formed at least 0.2 mm thinner than thickest facethickness of said high rigidity region (12).
 36. The metal golf clubhead according to claim 34 , wherein a region other than low rigidityregion (11) in said face member (9) is defined as a high rigidity region(12), and face thickness of said low rigidity region (11) is formed atleast 0.2 mm thinner than thickest face thickness of said high rigidityregion (12).
 37. The metal golf club head according to claim 33 ,wherein face thickness of said low rigidity region (11) is formed atleast 0.5 mm thinner than thickest face thickness of said high rigidityregion (12).
 38. The metal golf club head according to claim 34 ,wherein face thickness of said low rigidity region (11) is formed atleast 0.5 mm thinner than thickest face thickness of said high rigidityregion (12).
 39. The metal golf club head according to claim 33 ,wherein face thickness of said low rigidity region (11) is selected tobe 0.5 to 2.7 mm.
 40. The metal golf club head according to claim 34 ,wherein face thickness of said low rigidity region (11) is selected tobe 0.5 to 2.7 mm.
 41. The metal golf club head according to claim 35 ,wherein face thickness of said low rigidity region (11) is selected tobe 0.5 to 2.7 mm.
 42. The metal golf club head according to claim 36 ,wherein face thickness of said low rigidity region (11) is selected tobe 0.5 to 2.7 mm.
 43. The metal golf club head according to claim 37 ,wherein face thickness of said low rigidity region (11) is selected tobe 0.5 to 2.7 mm.
 44. The metal golf club head according to claim 38 ,wherein face thickness of said low rigidity region (11) is selected tobe 0.5 to 2.7 mm.
 45. The metal golf club head according to claim 31 ,wherein a material having a tensile strength of 100 kgf/mm² or greaterand having a Young's modulus of elasticity of 10000 kfg/mm² or lower isused for said face member (9).
 46. The metal golf club head according toclaim 32 , wherein a material having a tensile strength of 100 kgf/mm²or greater and having a Young's modulus of elasticity of 10000 kfg/mm²or lower is used for said face member (9).
 47. The metal golf club headaccording to claim 33 , wherein a material having a tensile strength of100 kgf/mm² or greater and having a Young's modulus of elasticity of10000 kfg/mm² or lower is used for said face member (9).
 48. The metalgolf club head according to claim 34 , wherein a material having atensile strength of 100 kgf/mm² or greater and having a Young's modulusof elasticity of 10000 kfg/mm² or lower is used for said face member(9).
 49. The metal golf club head according to claim 35 , wherein amaterial having a tensile strength of 100 kgf/mm² or greater and havinga Young's modulus of elasticity of 10000 kfg/mm² or lower is used forsaid face member (9).
 50. The metal golf club head according to claim 36, wherein a material having a tensile strength of 100 kgf/mm² or greaterand having a Young's modulus of elasticity of 10000 kfg/mm² or lower isused for said face member (9).
 51. The metal golf club head according toclaim 37 , wherein a material having a tensile strength of 100 kgf/mm²or greater and having a Young's modulus of elasticity of 10000 kfg/mm²or lower is used for said face member (9).
 52. The metal golf club headaccording to claim 38 , wherein a material having a tensile strength of100 kgf/mm² or greater and having a Young's modulus of elasticity of10000 kfg/mm² or lower is used for said face member (9).
 53. A metalgolf club head having a hollow portion, formed such that rigidity of aportion on a heel side (37) of a face portion (32) is made lower thanthat on a toe side (36).
 54. The metal golf club head according to claim53 , wherein rigidity of a portion excluding a vicinity of a face center(33) is lowered.
 55. The metal golf club head according to claim 53 ,wherein a region with lowered rigidity is defined as a low rigidityregion (35) and area of said low rigidity region (35) is selected to bein a range of 5% to 50% of face area.
 56. The metal golf club headaccording to claim 54 , wherein a region with lowered rigidity isdefined as a low rigidity region (35) and area of said low rigidityregion (35) is selected to be in a range of 5% to 50% of face area. 57.The metal golf club head according to claim 55 , wherein said lowrigidity region (35) is provided above face center (33).
 58. The metalgolf club head according to claim 56 , wherein said low rigidity region(35) is provided above face center (33).
 59. The metal golf club headaccording to claim 55 , wherein said low rigidity region (35) isprovided in a lower half region on a heel side (37).
 60. The metal golfclub head according to claim 56 , wherein said low rigidity region (35)is provided in a lower half region on a heel side (37).
 61. The metalgolf club head according to claim 53 , wherein when a region other thanlow rigidity region (35) of said face portion (32) is defined as a highrigidity region (40), face thickness of said low rigidity region (35) isselected to be thinner than face thickness of high rigidity region (40).62. The metal golf club head according to claim 54 , wherein when aregion other than low rigidity region (35) of said face portion (32) isdefined as a high rigidity region (40), face thickness of said lowrigidity region (35) is selected to be thinner than face thickness ofhigh rigidity region (40).
 63. The metal golf club head according toclaim 55 , wherein when a region other than low rigidity region (35) ofsaid face portion (32) is defined as a high rigidity region (40), facethickness of said low rigidity region (35) is selected to be thinnerthan face thickness of high rigidity region (40).
 64. The metal golfclub head according to claim 56 , wherein when a region other than lowrigidity region (35) of said face portion (32) is defined as a highrigidity region (40), face thickness of said low rigidity region (35) isselected to be thinner than face thickness of high rigidity region (40).65. The metal golf club head according to claim 57 , wherein when aregion other than low rigidity region (35) of said face portion (32) isdefined as a high rigidity region (40), face thickness of said lowrigidity region (35) is selected to be thinner than face thickness ofhigh rigidity region (40).
 66. The metal golf club head according toclaim 58 , wherein when a region other than low rigidity region (35) ofsaid face portion (32) is defined as a high rigidity region (40), facethickness of said low rigidity region (35) is selected to be thinnerthan face thickness of high rigidity region (40).
 67. The metal golfclub head according to claim 59 , wherein when a region other than lowrigidity region (35) of said face portion (32) is defined as a highrigidity region (40), face thickness of said low rigidity region (35) isselected to be thinner than face thickness of high rigidity region (40).68. The metal golf club head according to claim 60 , wherein when aregion other than low rigidity region (35) of said face portion (32) isdefined as a high rigidity region (40), face thickness of said lowrigidity region (35) is selected to be thinner than face thickness ofhigh rigidity region (40).
 69. The metal golf club head according toclaim 61 , wherein face thickness of said low rigidity region (35) isformed at least 0.2 mm thinner than face thickness of high rigidityregion (40).
 70. The metal golf club head according to claim 62 ,wherein face thickness of said low rigidity region (35) is formed atleast 0.2 mm thinner than face thickness of high rigidity region (40).71. The metal golf club head according to claim 63 , wherein facethickness of said low rigidity region (35) is formed at least 0.2 mmthinner than face thickness of high rigidity region (40).
 72. The metalgolf club head according to claim 64 , wherein face thickness of saidlow rigidity region (35) is formed at least 0.2 mm thinner than facethickness of high rigidity region (40).
 73. The metal golf club headaccording to claim 65 , wherein face thickness of said low rigidityregion (35) is formed at least 0.2 mm thinner than face thickness ofhigh rigidity region (40).
 74. The metal golf club head according toclaim 66 , wherein face thickness of said low rigidity region (35) isformed at least 0.2 mm thinner than face thickness of high rigidityregion (40).
 75. The metal golf club head according to claim 67 ,wherein face thickness of said low rigidity region (35) is formed atleast 0.2 mm thinner than face thickness of high rigidity region (40).76. The metal golf club head according to claim 68 , wherein facethickness of said low rigidity region (35) is formed at least 0.2 mmthinner than face thickness of high rigidity region (40).
 77. The metalgolf club head according to claim 53 , wherein a material having a lowerelasticity modulus than that of a material used in high rigidity region(40) is used for face portion (32) in said low rigidity region (35). 78.The metal golf dub head according to claim 54 , wherein a materialhaving a lower elasticity modulus than that of a material used in highrigidity region (40) is used for face portion (32) in said low rigidityregion (35).
 79. The metal golf club head according to claim 55 ,wherein a material having a lower elasticity modulus than that of amaterial used in high rigidity region (40) is used for face portion (32)in said low rigidity region (35).
 80. The metal golf club head accordingto claim 56 , wherein a material having a lower elasticity modulus thanthat of a material used in high rigidity region (40) is used for faceportion (32) in said low rigidity region (35).
 81. The metal golf clubhead according to claim 57 , wherein a material having a lowerelasticity modulus than that of a material used in high rigidity region(40) is used for face portion (32) in said low rigidity region (35). 82.The metal golf club head according to claim 58 , wherein a materialhaving a lower elasticity modulus than that of a material used in highrigidity region (40) is used for face portion (32) in said low rigidityregion (35).
 83. The metal golf club head according to claim 59 ,wherein a material having a lower elasticity modulus than that of amaterial used in high rigidity region (40) is used for face portion (32)in said low rigidity region (35).
 84. The metal golf club head accordingto claim 60 , wherein a material having a lower elasticity modulus thanthat of a material used in high rigidity region (40) is used for faceportion (32) in said low rigidity region (35).